1. Field of the Invention
The present invention relates to digital-to-analog converters. More specifically, the present invention is directed to a digital-to-analog converter having a variable speed incrementation of a converter output signal.
2. Description of the Prior Art
Digital-to-analog converters are well-known in the art and are widely used in contemporary process control systems to translate digital control signals from a digital computer to analog signals for operating process control devices, e.g., valves. However, it is often desired to manually override a computer control directive by an operator located at the field mounted analog control device. Such a situation may arise from a failure of the remote digital computer to correctly control the analog device due to an equipment failure. Further, it often necessary for an operator to make rapid gross adjustments of the analog device upon the discovery of the need to override the computer control. Such gross adjustments are ultimately not needed as the final desired analog control value is approached. Accordingly, a reduction in the speed of incrementation must be provided following the gross adjustment period. Such a variable speed analog signal incrementation will allow an operator to rapidly achieve a desired analog signal value with gross changes while preventing overshoot as the desired value is ultimately attained with a fine analog signal changes. A prior art system for achieving a variable speed of an analog output signal is shown in U.S. Pat. No. 3,826,991 of James A. Hogan. However, this prior art system has a continuously varying output signal which is difficult to control by an operator where small changes are desired and it is not directly applicable to a digital control system inasmuch as the varying output signal is generated as an analog signal to be summed with another fixed analog control signal.